Mail data can appear to be mirrored... when you set up the mirror I believe it will create the mail user ids on the mirror, and although mails can be sent to that server and put in the new mail box, none of the old messages sent to the original server will be there.

My concern if I used a shared device for the data, then that would be the single point of failure.

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Not if you ue e.g. glusterfs. Clusterfilesystems like glusterfs allow it to use a setup similar to raid over the network, where all files are stored on 2 or more servers incl. automatic repair of damaged shares. Glusterfs is better suited then rsync as it is near realtime. I would only use rsync when your server ar not connected to the internet with a fast connection e.g. if one server is in a datacenter and the other one hosted on a low mbit dsl line.

install the server part on both servers and also the client part on the same servers. So that every file gets stored on the local copy of the glusterfs store and additionally on the other server. So if one server goes down, he can still access all files.

You have to backup or move the data to a temporary directory and stop the services that access the data while you install the glusterfs share. After you mounted glusterfs to the /var/www or /var/vmail directory then copy the data back.